Metallicity Metallicity Dependent Wolf-Dependent Wolf-

Rayet windsRayet winds

PAUL CROWTHERPAUL CROWTHER

IntroductionIntroduction Wolf-Rayet (WR) stars represent the final Wolf-Rayet (WR) stars represent the final

phase in the evolution of very massive phase in the evolution of very massive stars prior to core-collapse;stars prior to core-collapse;

H envelope stripped away via (a) stellar H envelope stripped away via (a) stellar wind or (b) close binary evolution, wind or (b) close binary evolution, revealing products of either H-burning revealing products of either H-burning (WN) or He-burning (WC); (WN) or He-burning (WC);

WR stars show strong emission lines (HeII WR stars show strong emission lines (HeII 4686, CIV 58084686, CIV 5808) ) due to dense (10due to dense (10-5-5MMyryr-1-1), ), fast (<1,000 to >5,000 km sfast (<1,000 to >5,000 km s-1-1) winds ) winds

WR windsWR winds Denser winds Denser winds

than O stars, than O stars, so visual so visual

spectra of WR spectra of WR stars are stars are

dominated by dominated by emission emission

rather than rather than absorption absorption

lineslines

R(2/3)R(2/3)

>10>101212

>10>101111

ObservationsObservations WR spectroscopic signatures observed..WR spectroscopic signatures observed..

Individually within Local Group;Individually within Local Group; Knots within local star forming galaxies;Knots within local star forming galaxies; Average rest frame UV spectrum of LBGs.Average rest frame UV spectrum of LBGs.

H-depletion caused by winds during O & H-depletion caused by winds during O & LBV/RSG phases if single, so higher LBV/RSG phases if single, so higher threshold for WR formation at low Z. threshold for WR formation at low Z.

WRs indeed rarer at 1/5 ZWRs indeed rarer at 1/5 Z (SMC) vs Solar. (SMC) vs Solar.

Alternatively, H-depleted during close Alternatively, H-depleted during close binary evolution, without Z dependence. binary evolution, without Z dependence.

Dominant mechanism at low Z (e.g. IZw18)Dominant mechanism at low Z (e.g. IZw18)

Izw18Izw18

WN stars WN stars expected to expected to dominate in dominate in IZw18, but IZw18, but

solely solely signature of signature of

WC stars WC stars observed observed

(Brown et al. (Brown et al. 2002) 2002)

WR stars as GRB WR stars as GRB progenitorsprogenitors

Prime candidates for precursors of Type Prime candidates for precursors of Type Ib/c SNe & long/soft GRBs. Progenitors:Ib/c SNe & long/soft GRBs. Progenitors:

Associated with young massive stellar Associated with young massive stellar populations, populations,

Compact (excludes RSG progenitors),Compact (excludes RSG progenitors), Rapidly rotating core.Rapidly rotating core.

Primary challenge for single/binary GRB Primary challenge for single/binary GRB progenitors is requirement for rapid progenitors is requirement for rapid rotation at core-collapse (at Zrotation at core-collapse (at Z core core slowed down during RSG/WR phase).slowed down during RSG/WR phase).

Observed WR propertiesObserved WR properties WR wind WR wind

properties are properties are generally generally assumed to be assumed to be metallicity (Z) metallicity (Z) independent independent (Langer 1989). (Langer 1989).

Observational Observational trend to both trend to both earlier WN and earlier WN and WC subtypes at WC subtypes at low Z. Origin? low Z. Origin?

early late early lateearly late early late

Metallicity-dependent Metallicity-dependent winds?winds?

Wide scatter in WN mass-loss rates for Milky Wide scatter in WN mass-loss rates for Milky Way & LMC. Presence of hydrogen Way & LMC. Presence of hydrogen complicates picture (winds are denser if complicates picture (winds are denser if hydrogen is absent, Nugis & Lamers 2000). hydrogen is absent, Nugis & Lamers 2000).

However, winds of SMC WN winds However, winds of SMC WN winds are are weaker than similar H-rich LMC/Galactic weaker than similar H-rich LMC/Galactic stars (Crowther: Tartu workshop 2005).stars (Crowther: Tartu workshop 2005).

Trend to earlier WC subtypes in LMC vs Trend to earlier WC subtypes in LMC vs Milky Way was once believed to result from Milky Way was once believed to result from different C abundances. However, different C abundances. However, abundance pattern abundance pattern similarsimilar in both galaxies. in both galaxies.

WC metallicity WC metallicity dependencedependenceMilky Way WC

stars followed generic Nugis & Lamers (2000) calibration (red).

LMC stars followed similar relation (green), offset by -0.2 dex (Crowther et al. 2002).

Log(dM/dt) = 1.38 log(L/Lo) -12.35

Wind velocities?Wind velocities?

Limited Limited evidence in evidence in favour of Z- favour of Z- dependent dependent

wind velocities wind velocities except for except for

individual WO individual WO stars (= early stars (= early

WC). WC).

Theoretical evidence?Theoretical evidence? Radiatively driven wind model of Grafener Radiatively driven wind model of Grafener

& Hamann (2005), showed that Fe IX-XVII & Hamann (2005), showed that Fe IX-XVII lines initiate WC outflow.lines initiate WC outflow.

Vink & de Koter (2005) used Monte Carlo Vink & de Koter (2005) used Monte Carlo approach to investigate dM/dt approach to investigate dM/dt ZZ

dependence for WN stars (dependence for WN stars (=0.86; 10=0.86; 10-3-3 to to 1 Z1 Z) & WC stars () & WC stars (=0.66; Z=0.1-1 Z=0.66; Z=0.1-1 Z). ).

Origin of different exponents? C,N,O,Fe Origin of different exponents? C,N,O,Fe decrease in WN stars (CNO act as decrease in WN stars (CNO act as catalysts), but only Fe decreases in WC catalysts), but only Fe decreases in WC stars, due to primary C,O production. stars, due to primary C,O production.

Impact on WR subtypes?Impact on WR subtypes? High density High density

winds cause winds cause very efficient very efficient

recombination recombination from high to low from high to low

ions (shift ions (shift from`early’ from`early’

to`late’ to`late’ subtypes).subtypes).

Opposite is true Opposite is true for low density for low density

WR winds.WR winds.

earlyearly earlyearly

latelate latelate

Impact on WR Impact on WR populations?populations?

Effect of reduced WR wind densities at low Z:Effect of reduced WR wind densities at low Z: Increasingly weak-lined WR stars, as observed in Increasingly weak-lined WR stars, as observed in

SMC (more difficult to detect, especially WN stars);SMC (more difficult to detect, especially WN stars); Reduced emission line fluxes (factor of 3 to 20 at Reduced emission line fluxes (factor of 3 to 20 at

1/50Z1/50Z). Assumed Z-independent LMC/Milky Way ). Assumed Z-independent LMC/Milky Way calibration of Schaerer & Vacca (1998).calibration of Schaerer & Vacca (1998).

10 x lower emission line fluxes requires 10 x 10 x lower emission line fluxes requires 10 x more WR stars at low Z (Crowther & Hadfield more WR stars at low Z (Crowther & Hadfield 2006). Problematic for single star models.2006). Problematic for single star models.

Large WR populations at very low Z (e.g. IZw18) Large WR populations at very low Z (e.g. IZw18) likely to be binary evolution dominated.likely to be binary evolution dominated.

130 WN 20 WC

20 WC

LMC template stars required to derive reliable WR LMC template stars required to derive reliable WR populations in 0.5 populations in 0.5 ZZ starburst galaxy NGC3125. starburst galaxy NGC3125.

NGC3125-ANGC3125-A

Impact on ionizing Impact on ionizing fluxes?fluxes?

WR stars with WR stars with weakweak winds possess winds possess harderharder ionizing fluxes in Heionizing fluxes in He++ Lyman continua Lyman continua versus strong winds (Schmutz et al. versus strong winds (Schmutz et al. 1992;Smith et al. 2002)1992;Smith et al. 2002)

WR stars at low metallicity will possess ..WR stars at low metallicity will possess .. weak UV/optical spectral lines (hard to weak UV/optical spectral lines (hard to

directly detect via broad HeII 4686);directly detect via broad HeII 4686); strong H Lyman & Hestrong H Lyman & He++ Lyman continua (easy Lyman continua (easy

to detect indirectly via nebular HeII 4686). to detect indirectly via nebular HeII 4686). Strong nebular HeII in low-Z HII galaxies Strong nebular HeII in low-Z HII galaxies

from WRs & SNR (Izotov et al. 2006).from WRs & SNR (Izotov et al. 2006).

Impact on GRB Impact on GRB progenitors?progenitors?

Reduced WR mass-loss rates help to Reduced WR mass-loss rates help to maintain rapidly spinning core maintain rapidly spinning core through to core-collapse at low Z for through to core-collapse at low Z for single stars (Yoon & Langer 2005).single stars (Yoon & Langer 2005).

Reduced densities in immediate Reduced densities in immediate environment of GRBs with respect to environment of GRBs with respect to typical Milky Way WR stars, as typical Milky Way WR stars, as observed (Chevalier et al. 2004) observed (Chevalier et al. 2004)

SummarySummary Observational & theoretical evidence Observational & theoretical evidence

supports reduced wind densities & velocities supports reduced wind densities & velocities for low metallicity WR starsfor low metallicity WR stars

Addresses relative WR subtype distribution Addresses relative WR subtype distribution in Milky Way & Mag Clouds, & reduced WR in Milky Way & Mag Clouds, & reduced WR line strengths in SMCline strengths in SMC

Impacts upon WR populations at low Impacts upon WR populations at low metallicity as follows:metallicity as follows: Increased WR populations due to lower line Increased WR populations due to lower line

fluxes from individual stars;fluxes from individual stars; Harder ionizing fluxes; Harder ionizing fluxes; Low density GRB environment vs Solar Low density GRB environment vs Solar

counterpartscounterparts

IAU Symposium 250IAU Symposium 250 Title: “Massive Stars as Cosmic Engines”Title: “Massive Stars as Cosmic Engines”

Atmospheres of massive stars;Atmospheres of massive stars; Physics & evolution of massive stars; Physics & evolution of massive stars; Massive stellar populations in the nearby Universe; Massive stellar populations in the nearby Universe; Hydrodynamics and feedback from massive stars in galaxy Hydrodynamics and feedback from massive stars in galaxy

evolution; evolution; Massive stars as probes of the early UniverseMassive stars as probes of the early Universe

Venue: Kauai, HawaiiVenue: Kauai, Hawaii Dates: 10-14 December 2007Dates: 10-14 December 2007 SOC: P.Crowther (co-chair), M.Dopita, J. Fynbo, SOC: P.Crowther (co-chair), M.Dopita, J. Fynbo,

E.Grebel, T.Heckman, D. Hunter, G. Koenigsberger, E.Grebel, T.Heckman, D. Hunter, G. Koenigsberger, R. Kudritzki, N. Langer, A. MacFadyen, F. R. Kudritzki, N. Langer, A. MacFadyen, F. Matteucci, G. Meynet, A. Moffat, K. Nomoto, M. Matteucci, G. Meynet, A. Moffat, K. Nomoto, M. Pettini, J. Puls (co-chair)Pettini, J. Puls (co-chair)